Check the Presence of libattr1-dev in CMakeLists.txt

This adds (pedantic) checking for the include file `attr/xattr.h` and thus for the presence of `libattr1-dev`. Also all other required headers are now checked pedantically.

**Note:** The defined macro `look_for_include_files` will (pedantically) check for a list of given header files. For each header it finds, a variable like `HAVE_SYS_SOCKET_H` will be defined. This variable name is derived from the include file path by prepending it with `HAVE_`, replacing `/` and `.` with `_` and changing all characters to upper case.

**Note:** attr/xattr.h is broken in a sense that it requires the inclusion of `sys/types.h` before including `attr/xattr.h`. Since CMake creates a minimal C program that includes the requested header and checks if it compiles successfully, `attr/xattr.h` is reported to be *not found* even if it is there. I worked around that by pre-defining the include guard macro of `attr/xattr.h` namely `__XATTR_H__`. This still checks if `attr/xattr.h` is there, but essentially does not compile the *broken* header.